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Toulouse, France

Kis A.,Geodetic and Geophysical Institute | Kis A.,French National Center for Scientific Research | Agapitov O.,French National Center for Scientific Research | Agapitov O.,Taras Shevchenko National University | And 5 more authors.
Astrophysical Journal | Year: 2013

It is well known that shocks in space plasmas can accelerate particles to high energies. However, many details of the shock acceleration mechanism are still unknown. A critical element of shock acceleration is the injection problem; i.e., the presence of the so called seed particle population that is needed for the acceleration to work efficiently. In our case study, we present for the first time observational evidence of gyroresonant surfing acceleration in front of Earth's quasi-parallel bow shock resulting in the appearance of the long-suspected seed particle population. For our analysis, we use simultaneous multi-spacecraft measurements provided by the Cluster spacecraft ion (CIS), magnetic (FGM), and electric field and wave instrument (EFW) during a time period of large inter-spacecraft separation distance. The spacecraft were moving toward the bow shock and were situated in the foreshock region. The results show that the gyroresonance surfing acceleration takes place as a consequence of interaction between circularly polarized monochromatic (or quasi-monochromatic) transversal electromagnetic plasma waves and short large amplitude magnetic structures (SLAMSs). The magnetic mirror force of the SLAMS provides the resonant conditions for the ions trapped by the waves and results in the acceleration of ions. Since wave packets with circular polarization and different kinds of magnetic structures are very commonly observed in front of Earth's quasi-parallel bow shock, the gyroresonant surfing acceleration proves to be an important particle injection mechanism. We also show that seed ions are accelerated directly from the solar wind ion population. © 2013. The American Astronomical Society. All rights reserved. Source

Didelon P.,University Paris Diderot | Motte F.,University Paris Diderot | Tremblin P.,University Paris Diderot | Tremblin P.,University of Exeter | And 42 more authors.
Astronomy and Astrophysics | Year: 2015

Context. The surroundings of H ii regions can have a profound influence on their development, morphology, and evolution. This paper explores the effect of the environment on H ii regions in the MonR2 molecular cloud. Aims. We aim to investigate the density structure of envelopes surrounding H ii regions and to determine their collapse and ionisation expansion ages. The Mon R2 molecular cloud is an ideal target since it hosts an H ii region association, which has been imaged by the Herschel PACS and SPIRE cameras as part of the HOBYS key programme. Methods. Column density and temperature images derived from Herschel data were used together to model the structure of H ii bubbles and their surrounding envelopes. The resulting observational constraints were used to follow the development of the Mon R2 ionised regions with analytical calculations and numerical simulations. Results. The four hot bubbles associated with H ii regions are surrounded by dense, cold, and neutral gas envelopes, which are partly embedded in filaments. The envelope's radial density profiles are reminiscent of those of low-mass protostellar envelopes. The inner parts of envelopes of all four H ii regions could be free-falling because they display shallow density profiles: ρ(r) â r- q with \hbox{$q \leqslant 1.5$}. As for their outer parts, the two compact H ii regions show a ρ(r) â r-2 profile, which is typical of the equilibrium structure of a singular isothermal sphere. In contrast, the central UCH ii region shows a steeper outer profile, ρ(r) â r-2.5, that could be interpreted as material being forced to collapse, where an external agent overwhelms the internal pressure support. Conclusions. The size of the heated bubbles, the spectral type of the irradiating stars, and the mean initial neutral gas density are used to estimate the ionisation expansion time, texp ~ 0.1 Myr, for the dense UCH ii and compact H ii regions and ~ 0.35 Myr for the extended H ii region. Numerical simulations with and without gravity show that the so-called lifetime problem of H ii regions is an artefact of theories that do not take their surrounding neutral envelopes with slowly decreasing density profiles into account. The envelope transition radii between the shallow and steeper density profiles are used to estimate the time elapsed since the formation of the first protostellar embryo, tinf ~ 1 Myr, for the ultra-compact, 1.5-3 Myr for the compact, and greater than ~6 Myr for the extended H ii regions. These results suggest that the time needed to form a OB-star embryo and to start ionising the cloud, plus the quenching time due to the large gravitational potential amplified by further in-falling material, dominates the ionisation expansion time by a large factor. Accurate determination of the quenching time of H ii regions would require additional small-scale observationnal constraints and numerical simulations including 3D geometry effects. © ESO, 2015. Source

Waara M.,Swedish Institute of Space Physics | Nilsson H.,Swedish Institute of Space Physics | Stenberg G.,Swedish Institute of Space Physics | Andre M.,Swedish Institute of Space Physics | And 2 more authors.
Annales Geophysicae | Year: 2010

We present a case study of significant heating (up to 8 keV) perpendicular to the geomagnetic field of outflowing oxygen ions at high altitude (12 R E) above the polar cap. The shape of the distribution functions indicates that most of the heating occurs locally (within 0.2-0.4 RE in altitude). This is a clear example of local ion energization at much higher altitude than usually reported. In contrast to many events at lower altitudes, it is not likely that the locally observed wave fields can cause the observed ion energization. Also, it is not likely that the ions have drifted from some nearby energization region to the point of observation. This suggests that additional fundamentally different ion energization mechanisms are present at high altitudes. One possibility is that the magnetic moment of the ions is not conserved, resulting in slower outflow velocities and longer time for ion energization. © 2010 Author(s). Source

Zettergren M.,Embry - Riddle Aeronautical University | Semeter J.,Boston University | Burnett B.,Boston University | Oliver W.,Boston University | And 3 more authors.
Annales Geophysicae | Year: 2010

The work presents a data-model synthesis examining the response of the auroral F-region ion temperature, composition, and density to short time scale (<1 min) electric field disturbances associated with auroral arcs. Ion temperature profiles recorded by the Sondrestrom incoherent scatter radar (ISR) are critically analyzed with the aid of theoretical calculations to infer ion composition variability. The analyses presented include a partial accounting for the effects of neutral winds on frictional heating and show promise as the groundwork for future attempts to address ion temperature-mass ambiguities in short-integration ISR data sets. Results indicate that large NO+ enchancements in the F-region can occur in as little as 20 s in response to impulsive changes in ion frictional heating. Enhancements in molecular ion density result in recombination and a depletion in plasma, which is shown to occur on time scales of several minutes. This depletion process, thus, appears to be of comparable importance to electrodynamic evacuation processes in producing auroral arc-related plasma depletions. Furthermore, the potential of ionospheric composition in regulating the amounts and types of ions supplied to the magnetosphere is outlined. Source

Grant
Agency: National Science Foundation | Branch: | Program: STTR | Phase: Phase I | Award Amount: 150.00K | Year: 2007

This Small Business Technology TransferResearch (STTR) Program project aims to resolve if LEDs can provide properly formed light spectra and identify the appropriate LED control needed to resolve if COTS photodetectors can detect the full spectral character of analytes in-situ and identify the optimized sampling control needed and to innovate an LED(s) and photodetector(s) integrated functional design which achieves or exceeds the range and sensitivity of current spectrometer designs. This project will also help in the understanding of the cost to produce and build the business plan to market a micro-miniature spectrometer. with UW DMS. Develop committed business plan, based on resolving DFM issues. A spectrometer that significantly improves upon current specifications will be received well in markets ranging from remote oceanographic or atmospheric monitoring services to point-of-care diagnostic services in health care to national defense applications for water or airborne contaminant clouds. A micro-miniature spectrometer is proposed that will achieve half the size of current instruments, draw

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